In the field of high isolation broadband switching matrices, MMIC (microwave monolithic integrated circuit) type switches have been employed. Many high frequency switching circuits are analogous to electromechanical switching devices. The circuits have OFF and ON states which correspond to open and closed states of conventional switches, respectively. In the ON state, a switching circuit provides a low impedance pathway between an input and an output to facilitate propagation of a high frequency signal, e.g., a radio frequency (RF) signal. On the other hand, an OFF state opens the pathway to provide a high impedance or high reactance barrier against propagation of the signal. The isolation of the circuit is a quantitative measure of the degree to which the switching circuit prevents propagation of the high frequency signal, and is customarily given in decibels (dB). The isolation in the OFF state should preferably be as great as possible.
For instance, FIGS. 1 and 2 respectively show conventional single pole, single throw (SPST) and single pole, double throw (SPDT) switching circuits. The circuits each employ semiconductor switching elements shown as field effect transistors (FETs). FETs are commonly employed as voltage controlled resistors in which gate voltages control drain/source resistance, or as voltage controlled current sources in which gate voltages control drain and source currents. The FETs are configured either in shunt (source coupled to ground, labeled as 2) or in series (both drain and source coupled to other circuit elements or to a circuit input or output, labeled as 4). In either case, input control signals are provided to the gates of the FETs.
If a plurality of such switching circuits are coupled together for greater isolation, then the same control lines are coupled to each circuit. Therein lies a problem which tends to reduce isolation. High frequency signals have a way of leaking through any paths available to them, due to factors such as inductive coupling. This coupling takes place even when the circuit configuration would appear, on its face, to isolate the signals from the paths. In the cases of FIGS. 1 and 2, the high frequency signals propagating through the drains and sources of the series FETs leak through the control lines from one circuit to another, effectively reducing the "OFF" isolation. This problem is made worse since the circuits require pluralities of control inputs, two in the case of the SPST circuit and four in the case of the SPDT circuit.
The circuits of FIGS. 1 and 2 have additional undesirable design constraints. First, the control voltages must be approximately 0 and -5 volts. Since these are not standard logic voltages, additional voltage translating circuitry must be provided. Second, the sources of the shunt FETs require a low impedance path to ground.
Preparing a physical layout for high isolation switches has presented a related problem. If the transmission lines of a distributed type high isolation switch are folded alongside each other, inductive and capacitive coupling will usually cause degraded isolation. On the other hand, if the transmission lines are laid end-to-end in a straight line, the physical layout is more difficult to fabricate and package, and the result is in an elongated layout which results in less compactness of layout and lower manufacturing yield than is desirable. Also, it is sometimes desirable to use adjacent transmission lines of the switching circuit together with reactive or resistive circuit elements to provide feedback. In an elongated layout, a conductive bridge between two desired points in the circuit will probably be long enough to introduce undesirable phase shifts, thereby making it difficult to provide bridges between desired points to achieve desired effects.